Design and Characterization of MEMS BasedAccelerometers for Various Applications

R. Singh1, M. Singh2

1National Institute of Technology Karnataka, Surathkal, Karnataka, India2Indian Institute of Technology Delhi, New Delhi, Delhi, India

Abstract

Today, MEMS based accelerometers are used in a variety of applications. To name a few, theyare used in safety systems in automobiles, it has added a new dimension to miniaturization ofdevices, it has replaced traditional piezoelectric accelerometers, which were big and difficult touse. For its various applications, differing bandwidth (operating frequency range) and amplitudeof vibration are required. For example, the bandwidth for portable electronics is around 100-1000Hz; while for air bag application in automobile is around 1000-10000Hz. Thus, it becomesnecessary for us to study the design, principle and characteristic parameters involved in them. Nodoubt, experimental data are always useful and accurate. However, there always exists a need toconsult theoretical phenomena and mathematics, and FEA provides the best platform for this. In the present work, we have proposed three different designs for accelerometers. The designs(as shown in the Figure 1) have a rectangular proof masses suspended with crab-leg shapedcantilever supports. They are characterized using COMSOL Multiphysics® software. We didthe following two analyses to characterize our devices: (a.) Eigen Frequency Analysis (b.)Frequency Domain Analysis. The resonance frequency for the designs turns out to be 37 KHz,85 KHz and 135 KHz respectively in vacuum conditions (Figure 2). The correspondingbandwidths for the designs are found to be 10 KHz, 30 KHz and 40 KHz respectively. Theresults matched the experimental data, which was done on Laser Doppler Vibrometer. The characterization of the devices revealed that the devices can cater to varied applicationsranging from automobile to electronics. As we know, the characteristic parameters for thedevices are strong functions of the packaging conditions (pressure). Depending on the packagingconditions, the phenomenon called squeeze film damping influences characteristic parameterssuch as amplitude of oscillation, resonant frequency and band-width. This allows us to use themin different applications just by changing the packaging conditions. However, the changesobserved are very irregular and cannot be predicted. Thus, we simulated the designs for differentworking pressures. The resonant frequency versus pressure graphs for the designs are plotted(Figure 3) and results are experimentally verified. Finally, different packaging conditions aredetermined for different applications while taking care of other design parameters. Also, all theimportant characteristic parameters such as sensitivity, cross axis sensitivity, range, bandwidth,and resonance frequency are found for the devices. It is seen that the implementation of squeezefilm damping in FEA software results in a lot of computation. Thus, to cross check the values,we even proposed a novel way of modeling squeeze film damping in electrical domain using thecircuit comprising of parallel inductors and resistors. Finally, the values of simulation are used to

verify our new approach of modeling squeeze film. They matched in a very encouraging way.

Reference

Micro and Smart Systems, Prof. Ananthasuresh,Chapter 8- Integration of Micro and Smartsystems, Page 330, Year 2013

Squeeze film effects in MEMS Devices, Prof. R Pratap, S Mohite, Indian Institute of Science,Bangalore, India

Figures used in the abstract

Figure 1: The proposed and optimised designs of the accelerometers.

Figure 2: Eigen Frequency Analysis- Mode of vibration for the accelerometer designs.

Figure 3: Pressure versus Resonance Frequency Plot for the different designs.